The present invention relates to a device for inductively heating a billet with one or multi-layered billet heating coils and a method for inductively heating a billet with one or multi-layer billet heating coils.
Until now, billet heating assemblies of this type have a billet heating coil in single or multi-layered embodiments, a transport device for the heated billets or billet, and an electrical switching device for the temperature regulator. The billet-heating coil in such known devices comprises one or more galvanically separated zones. These are arranged sequentially such that the billet or billet support, upon the heating, is located completely in the zones of the billet-heating coil.
The electrical switching device supplies the Individual zones of the billet-heating coil with electrical energy via switching relays, such as furnace relays or Thyristor control elements. The switching relays, as well as the furnace relays and Thyristor control elements have a limited number of switching actions per unit of time. Thyristor control elements work friction-free, as opposed to the furnace relays.
The electrical energy, commonly supplied from the three-phase main supply network, is converted in the coil into an energy of the magnetic field with a determined output, and thus, through induction, is conveyed into the charge (billet or ingot). The energy of the magnetic field is converted in the billet into heat. The temperature is measured on the surface of the billet.
If the temperature at the measuring position lies under the provided desired temperature, the power of the associated zone is switched on by a temperature regulator. If the surfaces of the billet have reached the desired temperature, the power is switched off. With this two-point control, the existing power for supply is either switched on or completely switched off. In order to reduce the switching actions per unit of time of the switching organs, a temperature hysteresis is necessary with this type of control. The mains restoration takes place with a time difference only then (or in a moment) when the temperature on the surfaces of the billet goes below a provided value.
The temperature hysteresis of the two-point regulation has a large affect on the temperature accuracy of the warming on the billet. The abrupt switching on and off of the power causes network reactions in the form of inrush currents.
An affect of the radial temperature separations on the billet or billet (temperature difference between the core of the billet and their surfaces) is possible because of inertia only in a limited manner through the recovery or compensating time. Upon a turning off of the current, the billet endures during the recovery time either in the coil or externally in a compensating furnace.
The following disadvantages are associated with the above known devices:
the current-supplying network is not symmetrically loaded;
the switched-on current operates on the supplying network with a greater power/voltage as a result of the on/off switching;
the precision of the temperature regulator is impaired by the switching hysteresis. A smaller switching hysteresis for achieving a higher temperature effectiveness causes more switching action of the switching apparatus per unit of time, where the number of switching actions per unit of time of the switching apparatus, however, is limited;
no possibility exists for performing a thorough, uniform heating of the billet by the integration of the power division in application via frequency changes;
upon heating, the radial temperature gradients in the billet are always at their largest.
The present invention addresses the underlying problem of avoiding this inaccuracy and difficulty with the inductive billet heating with the goal of a precise construction of the temperature field in the billet for the most uniform and energy-saving radial and axial division of the temperature in the billet as possible, and therewith, a higher temperature accuracy and a better recurrence of the desired temperature profile in consideration of the permissible temperature gradient in the billet. In addition, the present invention provides the quickest and most efficient heating with a smaller energy consumption without requiring temperature measurement during the heating phase. The temperature should first be controlled after the warming.
This problem is solved with a device according to the present invention, in which the billet-heating coil is made up of multiple synchronically regulated zones relating to frequency and phase of the inductive field. A converter is provided for the current feed to each zone of the billet-heating coil with variable frequency and a modular construction, which is made up of a plurality of closed or self-contained power units with three-phase network feed and synchronization of phase and frequency of the output current.
The inductive billet-heating assembly is constructed with multiple zones, Z1 through Zn. It includes a multiple-zone and multi-layer billet-heating coil in a water-cooled form and a compensation-condenser connected thereto. A temperature measuring device is located in each zone, and indeed, pneumatically operation measuring points or an optical pyrometer T1 through Tn corresponding to the number of the n-zones (FIG. 2).
In addition, a converter having a modular construction is provided. All converter modules M1 through Mn form closed or self-contained power units. The three-phase network feed and synchronization of the phase and frequency of the output current is common for the modules.
The control takes place on an SPS-basis with a process visualization system with which the controller action of the converter module is implemented on the basis of a mathematical algorithm.
Next, the controller action of the converter module will be briefly described:
The power of zones Z1 through Zn of the billet-heating coil is regulated on the basis of the associated measured zone temperatures. For power regulation, the material value (and its temperature dependency), the geometry of the billet, and the energy-consumption ability of the billet (dP/dt) are included. The goal of the regulation is to achieve a specified temperature profile (in the tolerance region) in the shortest heating time, whereby these criteria determined simultaneously the maximal efficiency of the heating.
In order to realize the above goals, the control of the optimal frequency for the operation of the multi-layered inductive billet-heating coil is determined. The limiting value for the temperature dependent temperature gradients in the billet (input) limit the timely development of the measured temperature on the billet surfaces. An answer-back signal via the actual temperature gradients in the billet and the temperature on the surface of the billet allows the temperature field in the billet to be determined.
The method is applied in connection with multi-layer billet-heating coils and a converter.
For inductive billet heating, an inductive billet-heating assembly serves round billets made of copper, aluminum, and their alloys, as well as iron and austenitic materials of larger diameters.
The current feed takes place by means of a converter.
the converter has a modular construction;
the modules are synchronized (frequency and phase of the field);
the frequency is variable;
the output quantities of the converter (voltage, current) are sinus-shaped;
the load or charge of the current network is symmetrical, independent from the number of connected zones of the billet-heating coil; and
the noise production in the assembly is reduced by means of a specialized control algorithm of the power electronics.
The billet-heating call is a multi-layer embodiment comprising multiple zones. The individual zones are with respect to the power supply independently supplied with energy, namely, individual via corresponding converter module. The current feed of all zones is synchronized in frequency and phase of the field produced.
The frequency of the feed voltage (of the current) is variable in a wide area and is regulated during the heating of the billet. The regulation of the power of the individual zones of the billet-heating coil rests on a mathematical model, which considers the weight, the material characteristics, the temperature on the surface of the billet, and the timely development of this temperature. In this manner, the following features of the heating are achieved:
a method for quickly and inductively heating the billet is combined with a good, uniform through-heating;
an energy-savings is provided by means of the adjustment of the frequency of the current at the optimal value in dependence on the billet diameter, the alloy of the billet and the temperature, and indeed, under minimizing of the coil waste, as well as optimizing of the division of the energy sources in the billet;
consideration of the thermally limited mechanical voltages in the billet of special alloys with the shortest heating times.